1. Field of the Invention
The present invention relates to a clock generation circuit having a redundant structure composed of two systems, and more particularly to the detection of frequency fluctuation caused by age deterioration of the oscillator that serves as the clock source.
2. Description of the Related Art
From the prior art, clock generation circuits have been considered that generate mutually synchronized clocks in two systems: an active system and a standby system. Such a circuit is described in, for example, JP-A-1991-272234. In this type of clock generation circuit, the active system and standby system each has a clock source, whereby the clock generation circuit generates clocks using clocks generated in one or the other system.
Highly stable oscillators are used as the clock sources of this type of clock generation circuit. Because frequency fluctuation occurs in such oscillators with the passage of time, a reference signal from an external unit such as a GPS or radio clock is typically used for constant correction.
However, in a system where the input of a reference signal from an outside source is not possible, the fluctuation in frequency that occurs with age deterioration cannot be automatically corrected. As a result, the problem exists that time and effort must be expended to detect whether fluctuation in frequency has occurred due to age deterioration in the oscillators by carrying out a frequency measurement when adjusting frequency during periodic maintenance. The problem further exists that the occurrence of fluctuation in frequency in oscillators can be checked only during periodic maintenance, and an accurate clock therefore cannot be generated until such maintenance.